1. Field of the Invention
The present invention relates to a position measuring system which has an interferometer to measure the position of a measurement target using interference of laser beams. The present invention also relates to an exposure apparatus having such a position measuring system.
2. Description of the Related Art
A laser interferometer having a helium-neon (He—Ne) laser as a light source is used for accurate length measurement or coordinate measurement. A measuring apparatus using a laser interferometer of this kind must include a wavelength compensation unit to compensate for a change in the measured optical path length caused by a change in the refractive index of an atmospheric gas through which a laser beam passes. A wavelength compensation unit generally includes a wavelength detection unit and a wavelength calculation unit. A wavelength detection unit generally employs a technique of detecting a wavelength using sensors to measure the atmospheric pressure, temperature, and humidity, or a technique of detecting a wavelength by using a wavelength sensor using an interferometer.
FIG. 4 is a view showing a position measuring system having a wavelength compensation unit described in Japanese Patent No. 02650953. Laser interferometers 23 and 24 for position measurement measure the position of a stage 1 as a measurement target using a laser beam from a laser light source 21. The laser beam from the laser light source 21 is partially directed to a wavelength sensor 10. The wavelength sensor 10 monitors the wavelength change of the laser beam.
In the position measuring system described in Japanese Patent No. 02650953, the wavelength sensor 10 is located far from the laser optical paths of the laser interferometers 23 and 24. This is because it is difficult from the viewpoint of space and cost to place the wavelength sensor 10 near the laser optical paths of the laser interferometers for position measurement.
However, when the wavelength sensor 10 is installed far from the laser optical paths of the laser interferometers 23 and 24, a difference is generated between the atmosphere in the wavelength sensor 10 and the atmosphere in the laser optical paths of the laser interferometers 23 and 24 for position measurement, resulting in a large compensation error. The present inventor found that especially, e.g., a fan acts as a sound source and generates a sonic wave (aerial vibration), and the difference between the phase of the sonic wave in the laser optical paths of the laser interferometers 23 and 24 and that at the wavelength detection position of the wavelength sensor 10 affects the compensation error.